Apparatus and method for floating-zone melting of a semiconductor rod

ABSTRACT

This invention provides a device for the floating-zone melting of a semiconductor rod. The device comprises a treatment container having means to mount a semiconductor rod therein, a resonant circuit capacitor slidably mounted and sealed in an opening in a wall of the treatment container, said resonant circuit capacitor being a part of an electrical circuit resonantor, connecting wires extended from said resonant capacitor to a high frequency generator outside of the treatment container, and a single-wound induction heating coil movably mounted within said treatment container and arranged to be concentrically positioned around said semiconductor rod, said heating coil being connected at one end to the inner conductor of the resonant capacitor and at the other end to the outer conductor of said capacitor.

United States Patent 1191 Stut et al. Oct. 30,1973

[54] APPARATUS AND METHOD FOR 3,484,679 12/1969 Hodgson et a] 317 242 xFLOATINGJONE MELTING OF A 3,339,130 8/1967 Schaeffer 317/242 XSEMICONDUCTOR ROD Inventors: Hans Stut, Grobenzell; Wolfgang Keller,Munich, both of Germany Assignee: Siemens Aktiengesellschaft, Berlin andMunich, Germany Filed: Aug.'l0, 1972 Appl. No.: 279,482

Foreign Application Priority Data Aug. 17,1971 Germany P 2141 188.1

References Cited UNITED STATES PATENTS 6/1972 Keller 219/1075 9/1959Emeis et al.

....... 13/DlG. 1

Primary Examiner-R. F. Staubly Assistant Examiner-B. A. ReynoldsAttorneyBenjamin H. Sherman et al.

[57] ABSTRACT This invention provides a device for the floating-zonemelting of a semiconductor rod. The device comprises a treatmentcontainer having means to mount a semiconductor rod therein, a resonantcircuit capacitor slidably mounted and sealed in an opening in a wall ofthe treatment container, said resonant circuit capacitor being a part ofan electrical circuit resonantor, connecting wires extended from saidresonant capacitor to a high frequency generator outside of thetreatment container, and a single-wound induction heating coil movablymounted within said treatment container and arranged to beconcentrically positioned around said semiconductor rod, said heatingcoil being connected at one end to the inner conductor of the resonantcapacitor and at the other end to the outer conductor of said capacitor.

12 Claims, 2 Drawing Figures PATENTED OCT 30 I975 APPARATUS AND METHODFOR FLOATING-ZONE MELTING OF A SEMICONDUCTOR ROD BACKGROUND OF THEINVENTION Field of the Invention This invention relates to a method anddevice for the floating-zone melting of a semiconductor rod. Moreparticularly, this invention relates to a device having a single-woundinduction heating coil arranged in a treatment container to coaxiallymove with respect to a semiconductor rod mounted therein.

Generally, such devices that are used for the floatingzone melting of arod such as a semiconductor rod are disclosed and described in GermanPat. DAS 1,188,043 and German Pat. DOS 1,444,530. Such devices have ahigh frequency source which is arranged outside the treatment containerin which the semiconductor rod is to be treated by zone melting and aninduction heating coil which is also arranged inside the treatmentcontainer. The heating coil generally is slidable in a parallel mannerto the axis of the mounted semiconductor rod within the treatmentcontainer. In such devices, the connecting cable or wire from theheating coil is generally placed through an electrical passage in thewall of the treatment container for the electric connection between thehigh frequency generator and the induction heating coil, whereby theinduction heating oil is connected at both ends to the high frequencygenerator through the same electrical passage.

The conneting electrical circuit, i.e., the connecting cable and thepassages as well as the induction heating coil, which is effectivebetween the high frequency generator and the semiconductor rod, does nothave an alternating current which is exclusively real. This is truebecause the electrical current which is induced by the effect of theelectrical field of the induction heating coil in the semiconductor rod,contains a power component as well as a reactive component. However, dueto the presence of the reactive component, the degree of efficiency ofthe heating is decreased. Thus, there is a need to reduce the reactivecomponent as much as possible.

SUMMARY OF THE INVENTION In view of the disadvantages of prior devicesfor zone melting semiconductor rods, the present inventionprovides animproved device for the floating-zone melting of semiconductor rodswhich is e'fficient,'practical and effective.

According to the present invention, a device is provided which isdesigned to reduce the reactive component of the electrical currentwhich is induced by the effect of the electrical field of the inductionheating coil. For this purpose, the present device includes a capacitorarranged in parallel to the heating coil to form an electrical resonantcircuit. Since the reactive components of the current are primarily dueto the induction coating of the supply lines between the high frequencygenerator and the induction heating coil, a reduction of the reactiveimpedance of the circuit as well as a reduction of the reactivecomponent of the heating current which is induced in the treated rod isobtained by the parallel arrangement of the capacitor in the device. Theparallel capacitor .is purposely arranged in proximity of the inductionheating coil so that it is protected against the radiation of the hotmelting zones which are necessary in melting the semiconductor rod.Also, the capacitance of the capacitor should be sufficiently high inorder to compensate and overcome the reactive component of the electriccurrent induced by the induction heating coil.

According to the present invention it is advantageous to have the supplyline or connecting wires from the high frequency generator to thecapacitor as well as the capacitor itself self-inductive in order toexert a certain influence on the characteristics of the electricalresonant circuit and to increase the entire inductance of the electricalresonant circuit.

The present device for the floating-zone melting of a semiconductor rod,comprises a treatment container having means to mount therein asemi-conductor rod, a resonant circuit capacitor slidably mounted andsealed in an opening in a wall of the treatment container and forming apart of an electrical resonant circuit, connecting wires extending fromthe capacitor to a high frequency generator outside of said treatmentcontainer, and a single-wound induction heating coil movably mountedwithin said treatment container, said heating being arranged to beconcentrically positioned around said rod within said treatmentcontainer and connected at one end to the inner con-ductor of theresonant capacitor and at the other end to the outer conductor of thecapacitor.

It therefore, is an object of the present invention to provide animproved device for the floating-zone melting of a semiconductor rod.

A more specific object of the present invention is to provide a devicefor the floating-zone melting of a semiconductor in which the reactivecomponent of the electric current induced by the heating coil in thesemiconductor rod, is reduced to a minimum.

Other objects, features and advantages of the invention will be readilyapparent from the following description of certain preferred embodimentsthereof, taken in conjunction with the accompanying drawings, althoughvariations and modifications may be effected without departing from thespirit and scope of the novel concepts of the disclosure.

BRIEF DESCRIPTION OF DRAWINGS DETAILED DESCRIPTION OF THE PREFERREDEMBODIMENTS Referring to FIG. 1, there is schematically shown a deviceembodying the present invention. The device, generally referred to bythe numeral 20, is primarily useful for the floating-zone melting of arod such as a semiconductor rod 4. The device, as shown, includes acylindrical treatment container 1 which has mounting means 2 and 3 formovably and rigidly mounting as desired, the semiconductor rod 4 withinthe treatment container. The mountings 2 and 3 may be arranged to beaxially shiftable with respect to each other. Mounted within an opening22 in the bottom wall or end cover 24 of the treatment container 1 is aresonant circuit ca pacitor 5 which is slidably mounted but sealedwithin the opening 22. As shown in FIG. 2, the resonant capacitor is arigid coaxial line which can be moved vertically upward and downward ina parallel manner with respect to the semiconductor rod 5 within thetreatment container 1. The resonant capacitor 5 is supported in theopening 22 at the passage point 17 through the bottom cover 24 of thetreatment container l by respectively sealed passages 26 and 28 in sucha way that the axial shifting of the resonant circuit capacitor 5 orvertical movement of the resonant circuit capacitor 5 at the passagepoint 17 is possible with out any outer air being admitted into thetreatment container 1. Thus, there is provided in effect, a vacuumwithin the treatment container.

As illustrated in FIGS. 1 and 2, a single-wound induction heating coil 6is connected to the resonant circuit capacitor 5 within the treatmentcontainer 1. The induction heating coil 6 is arranged to be coaxiallypositioned around the semiconductor rod 4 and to be axially shifted withrerespect to the rod 4 which is to be treated.

The resonant circuit capacitor 5 may be merely a mounting or holder forthe heating induction coil 6, although it is preferred to have theholding or mounting means for the heating coil 6 as a resonant circuitcapacitor to provide an electrical resonant circuit as describedhereinbelow. In such case, as shown in FIG. 2, the resonant capacitor 5may only consist of an inner conductor 7, a tube-saped massive capacitordielectric l3 and an outer conductor 8. Both the inner 7 and outer 8conductors can be connected directly at their upper ends with therespective ends of the singlewound induction heating coil 6.

.As shown in FIG. 1, the resonant capacitor 5 is connected by means of aconnecting line or coaxial cable 9 to a high frequency generator 10located outside of the treatment container 1. The connecting line 9 ispreferably arranged as a coaxial cable. The connecting line 9, however,is flexible whereas the resonant circuit capacitor 5 is rigid and solid.The connecting line 9 can be made of a common dielectric plasticmaterial such as polyethylene or polytetrafluoroethylene. The relativecapacitance per meter of the coaxial cable 9 made of such material isbetween about 30 and 100 picofarads. r

During the floating-zone melting of the semiconductor rod 4 which ispreferably made of silicon, the frequency of the alternating currentwhich is supplied by the high frequency generator 10 is adjusted toapproximately 3 to 4 Mega-Hertz to provide the optimum heating effect.Because of the inductance of the heating coil which is provided in thecase of the common high frequency connecting line 9, a resonant circuitcapacitance of at least 20 nano-farads, and preferably between 30 and 40nano-farads, would be needed'to effectively reduce the reactivecomponents of the electrical current induced by the heating inductioncoil 6 in the semiconductor rod which is to be zone melted. Thiscapacitance is obtained without the simultaneous increase of inductanceof the heating coil.

It is noted that if a conventional coaxial cable would be used as aresonant circuit capacitor, great lengths of this coaxial cable would beneeded to perform the function of the resonant circuit capacitorillustrated in the drawings. Accordingly, this would provide aconsiderable increase of the inductance of the entire electricalresonant circuit and also increase the undesired reactive components ofthe electrical current induced by the heating coil in the semiconductorrod.

However, according to the present invention, by using a massivecapacitor dielectric 13 with a relative dielectric constant of at least10, and in some cases a dielectric constant of at least 100, the desiredmelting would be achieved so that the capacitance of such a coaxialcable would increase by the same factor whereby the relative inductanceof the heating coil would not be affected or increased. Accordingly, itis preferred to use ceramics as the resonant capacitor dielectric suchas a titanic oxide or magnesium titanic which are especially well suitedas the capacitor dielectric. Thus, the required resonant circuitcapacitance with an inductively low coaxial capacitor 5 can be easilyobtained, whereby the length of the coaxial capacitor 5 is no largerthan that of the minimum length of the semiconductor rod 4 which isrequired for zone melting.

The coaxial capacitor 5 as illustrated in FIG. 2, can be constructed byhaving an outer coaxial metal tube 14. This tube then will provide themetal protection of the capacitor since it can be slid in the samedirection as the inner conductor 7 whereas the capacitor 5 itself can berigidly arranged in this tube 14. A further development of this type ofcapacitor can be'provided as disclosed in the German specification DOS1,916,316. In this specification there is described a device forinductive floating-zone melting of rods with an induction heating coilwhich is arranged in a closed off treatment container for creating andheating the melting zone in the rods, to be treated. Th inductionheating coil is connected by means of an electrical supply line with anenergy source that is arranged outside the treatment container. In thisconstruction, the part of the electrical supply line which is passedthrough the wall of the treatment container consists of severaltube-shaped single conductors which are arranged in a coaxial way toeach other. The tube-shaped single conductors are divided into twogroups in such a manner that each group consists of single conductorswhich are connected in parallel to each other and each single conductorincludes only a single conductor of the other group. Preferably, theouter tube of these tube-shaped conductors forms with its adjacentconductor the resonant circuit capacitor which is fixed within the othertube-shaped capacitor coaxially by suitable means; for example, by meansof a massive dielectric having a high dielectric constant. By such aparallel connection of the conductors of the resonant capacitor, theinductance as well as the capacitance of the entire electrical resonantcircuit is increased. Thus, by this construction there is providedaccording to the present invention another embodiment of a device whichreduces the reactive components of the electrical current induced by theheating coil in the semiconductor rod.

In the preferred embodiment according to the present invention, themounting 5 for the induction heating coil 6 is the resonant circuitcapacitor that is concentrically positioned inthe interior of thecylindrical metal tube 14.,As shown in FIG. 2, the resonant circuitcapacitor 5 is thus formed with a ceramic tube 13 having a largedielectric constant, and with a metal layer as the inner conductor 7 onthe inner wall of the tube 13 and an insulating metal layer as the outerconductor 8 on the outer wall of the ceramic tube 13. The metal tube 14which surrounds the capacitor is designed as a container for watercooling means 14 and 15a. For this purpose the tube 14 is closed off atits upper and lower ends with the exception of the entries and exits forthe cooling means and 15a. As shown in FIG. 2, this capacitor ispermeated by the body of the actual resonant circuit capacitor in itsentire length in such a way that the interior of the container consistsof two eccentric chambers which are separatedfrom each other by thecapacitor. One of these chambers is located in the interior of the tubeshaped capacitor member whereas the other chamber is enclosed on theoutside by the tube 14 and on the inside by the tube shaped capacitormember. Both chambers are provided with two connection pipes 16 and 16afor the respective cooling means 15 and 15a. The cooling means reachesthe inner chambers for instance by means of the lower termination orsealing ends 12 of the resonant capacitor 5 for the induction heatingcoil 6 from where it goes by means of the upper terminal 11 carrying theinduction coil 6 by means of the outer chamber of the resonant capacitor5.

The capacitor 5 is constructed so that the ceramic tube 13 insulateselectrically the two parts of the metallic terminals 11 and 12 as wellas the inner 7 and outer 8 conductors, in respect to each other. Thecontacting of the conductors 7 and 8 of the capacitors is provided bymeans of the coaxial cable 9, as indicated above, whereby the cable 9contacts electrically by means of the outer part of the lower terminal12, the outer conductor 8, and by means of the inner part of theterminal 12, the inner conductor 7 of the resonant capacitor.

The inner conductor7 constitutes the electric connection to the innerpart of the terminal 11 which together form one electrical connectionwith the induction coil 6, while the other electrical connection of theinduction coil 6 is formed by the outer conductor 8 and possibly themetallic outer conductor tube 14 as well as the outer part of theterminal 11.

The inner and outer components of the terminals 11 and 12 may beconnected with the capacitor member as well as with the metal tube 14,the induction coil 6 and the ends of the connecting cable 9 byselfsoldering. In addition, the terminals, particularly terminal 11, maybe sealed with a protective layer (not shown) consisting of heatresistor material such as silicon or polytetrafluoroethylene.

The treatment container 1 is preferably made of a stable material suchas quartz. The inner conductor 7 and the outer conductor 8 of thecapacitor may for instance consist of silver which is burned onto thesurface of the ceramic tube 13.

The passage or contact point 17 of the resonant capacitor 5 in theopening 22 of the bottom cover 24 of the treatment container 1, may be agasket which can consist, for example, of several ring-shaped gaskets 29(two shown) which are arranged in a stack and are tightly connected atthe passage point with the cover 24 of the treatment container 1. Thespaces between the adjacent gaskets can be filled with a suitable liquidsealing means 30 which should be temperature resistant and as little aspossible, volatile.

It is noted that it is also possible to accommodate two or severalparallel-connected resonant capacitors 5 within the outer tube 14. Inthis case, a concentric or eccentric arrangement of both capacitors ispossible. However, it has been found that the capacity which can beachieved by a coaxial capacitor of to centimeters in length issufficient to effectively reduce the reactive components of theelectrical current induced by the heating coil 6 in the semiconductorrod 4, and to substantially increase the effective heating and meltingthereof.

It is clear from the above description of the present invention that animproved device is provided for the effective floating-zone melting of asemiconductor rod in that it is constructed to substantially reduce thereactive component of the electric current induced by the heating coil 6in the semiconductor rod.

Although minor modifications might be suggested by those versed in theart, it should be understood that we wish to embody within the scope ofthe patent awarded hereon all such modifications as might reasonably andproperly come within the scope of our contribution as defined by theappended claims.

We claim as our invention:

1. A device for the floating-zone melting of a semiconductor rod whichcomprises:

a treatment container having means for mounting therein a semiconductorrod;

a resonant circuit coaxial capacitor having an inner conductor means andan outer conductor means and slidably mounted and sealed in an openingin a wall of the treatment container;

a high frequency cable including connecting wires extended from saidcapacitor to a high frequency generator positioned outside of thetreatment container, said wires connected to respective ones of saidconductor means; and

a wound induction heating coil movably mounted in said treatmentcontainer, said heating coil being arranged to be concentricallypositioned around said semiconductor rod and connected at one end to theinner conductor means of the resonant capacitor and at the other end tothe outer conductor means of said capacitor.

2. A device according to claim 1, wherein the resonant capacitor has acapacitance of at least 20 times as large as the capacitance of theconnecting wires extended from the capacitor to the high frequencygenerator. v

3. A device according to claim 2, wherein the induction heating coil ismounted only on the resonant circuit capacitor.

4. A device according to claim 1, wherein said inductive heating coiland resonant circuit capacitor are axially shiftable with respect tosaid semiconductor rod in the treatment container.

5. A device according to claim 1, wherein the resonant circuit capacitoris concentric with at least one other concentrically arranged metaltube.

6. A device according to claim 1, wherein the dielectric constant of themassive dielectric of the resonant circuit capacitor is at least 10times larger than the dielectric constant of the insulation of theconnecting wires to the high frequency generator.

7. A device according to claim 1, comprising a gasket in the wall ofsaid treatment container, said outer conductor means of the resonantcircuit capacitor passing through said gasket, said gasket consisting ofseveral ring-shaped gaskets which are tightly and sealably connectedwith the wall of the treatment container whereby the spaces between saidring shaped gaskets are filled with an inert liquid sealant.

8. A device according to claim 7, wherein the dielectric of the resonantcircuit capacitor consists of a ceramic having a high dielectricconstant.

9. A device according to claim 8, wherein the dielectric is composed ofmaterial selected from the group consisting of titanium dioxide andmagnesium titanic.

10. A device according toclaim 8, wherein the resonant circuit capacitorcomprises a tube of insulating material having an inner conductor at itsinner surface and an outer conductor at its outer surface, said innerand outer conductors being arranged in the form of two metal layerswhich are insulated with respect to each other.

11. A device according to claim 10, herein the resonant circuitcapacitor is enclosed in a metal tube which is closed off at both endsby terminals in a manner so that the inner space between the metal tubeand capacitor is divided into two separate chambers which are incommunication with each other by means of said induction heating coil.

12. A device according to claim 11, wherein said chambers are arrangedwith one having an entry and the other an exit for the passage of acooling means through said resonant circuit capacitor.

1. A device for the floating-zone melting of a semiconductor rod whichcomprises: a treatment container having means for mounting therein asemiconductor rod; a resonant circuit coaxial capacitor having an innerconductor means and an outer conductor means and slidably mounted andsealed in an opening in a wall of the treatment container; a highfrequency cable including connecting wires extended from said capacitorto a high frequency generator positioned outside of the treatmentcontainer, said wires connected to respective ones of said conductormeans; and a wound induction heating coil movably mounted in saidtreatment contAiner, said heating coil being arranged to beconcentrically positioned around said semiconductor rod and connected atone end to the inner conductor means of the resonant capacitor and atthe other end to the outer conductor means of said capacitor.
 2. Adevice according to claim 1, wherein the resonant capacitor has acapacitance of at least 20 times as large as the capacitance of theconnecting wires extended from the capacitor to the high frequencygenerator.
 3. A device according to claim 2, wherein the inductionheating coil is mounted only on the resonant circuit capacitor.
 4. Adevice according to claim 1, wherein said inductive heating coil andresonant circuit capacitor are axially shiftable with respect to saidsemiconductor rod in the treatment container.
 5. A device according toclaim 1, wherein the resonant circuit capacitor is concentric with atleast one other concentrically arranged metal tube.
 6. A deviceaccording to claim 1, wherein the dielectric constant of the massivedielectric of the resonant circuit capacitor is at least 10 times largerthan the dielectric constant of the insulation of the connecting wiresto the high frequency generator.
 7. A device according to claim 1,comprising a gasket in the wall of said treatment container, said outerconductor means of the resonant circuit capacitor passing through saidgasket, said gasket consisting of several ring-shaped gaskets which aretightly and sealably connected with the wall of the treatment containerwhereby the spaces between said ring-shaped gaskets are filled with aninert liquid sealant.
 8. A device according to claim 7, wherein thedielectric of the resonant circuit capacitor consists of a ceramichaving a high dielectric constant.
 9. A device according to claim 8,wherein the dielectric is composed of material selected from the groupconsisting of titanium dioxide and magnesium titanic.
 10. A deviceaccording to claim 8, wherein the resonant circuit capacitor comprises atube of insulating material having an inner conductor at its innersurface and an outer conductor at its outer surface, said inner andouter conductors being arranged in the form of two metal layers whichare insulated with respect to each other.
 11. A device according toclaim 10, wherein the resonant circuit capacitor is enclosed in a metaltube which is closed off at both ends by terminals in a manner so thatthe inner space between the metal tube and capacitor is divided into twoseparate chambers which are in communication with each other by means ofsaid induction heating coil.
 12. A device according to claim 11, whereinsaid chambers are arranged with one having an entry and the other anexit for the passage of a cooling means through said resonant circuitcapacitor.